1. Field of the Invention
The invention concerns an apparatus for producing colour value signals which correspond to a desired arrangement of colour bars.
2. Description of the Prior Art
In the book "Farbfernsehen 2," by Helmut Schonfelder, pages 212 to 213, there is disclosed a method of producing video frequency colour bar signals wherein a sequence of keying pulses at horizontal frequency is derived by means of a chain of bistable flip-flop stages. The keying pulses can be connected through a switching field to distribution stages. Because a predetermined colour is allocated to each distribution stage, certain colour bar sequences can be selected according to the circuit connections. For comparative tests the European Broadcasting Union (E.B.U.) has laid down as a standard colour bar test a colour bar sequence arranged in descending brightness values. In this test the level of a luminance bar (white) is 100%. The levels of the colour bars are reduced to 75%. The level reduction of the colour bars was formerly effected by pulsed voltage dividers. The generation of the necessary keying pulse signal is very costly in circuitry. For the video frequency color bar signals R. G and B a level deviation of only .+-. 0.5% is permissible. The maintenance of this tolerance requires the use of expensive temperature compensated circuits which have to be adjusted in time-consuming operations.
It is an object of the invention to provide an alternative means of producing colour bars which avoids these disadvantages.
According to the present invention there is provided an apparatus for producing video frequency color value signals which correspond to a desired arrangement of vertical colour bars on a television screen, comprising means for producing during each of a plurality of successive line periods a series of signals having a separation less than line period, means for applying each series of signals so produced to a sequence generator adapted to generate a predetermined sequence of sets of binary signals in response to each series of signals, each set of binary signals being generated in response to a respective one of the series of signals, a signal translator adapted to provide a predetermined sequence of further sets of binary signals in response to each sequence of firstmentioned sets of binary signals, each further set of binary signals being produced in response to a respective one of the firstmentiond sets and having binary values determined by the values of the said respective firstmentioned set, and a digital-to-analogue converter adapted to derive from each sequence of further sets of binary signals a sequence of sets of color value signals, the sets of colour value signals corresponding to the colours of the desired colour bars.
The invention further provides an apparatus for producing video frequency colour value signals which correspond to a desired arrangement of horizontal colour bars on a television screen, comprising means for producing during each of a plurality of successive field periods a series of signals having a separation greater than the line period, means for applying each series of signals so produced to a sequence generator adapted to generate a predetermined sequence of sets of binary signals in response to each series of signals, each set of binary signals being generated in response to a respective one of the series of signals, a signal translator adapted to provide a predetermined sequence of further sets of binary signals in response to each sequence of firstmentioned sets of binary signals, each further set of binary signals being produced in response to a respective one of the firstmentioned sets and having binary values determined by the values of the said respective firstmentioned set, and a digital-to-analogue converter adapted to derive from each sequence of further sets of binary signals a sequence of sets of colour value signals, the sets of colour value signals corresponding to the colours of the desired colour bars.
By the use of the invention it is possible to generate in a simple manner colour value signals for vertically or horizontally arranged colour bars.